The invention relates to a method of providing electrical contact to a semiconductor cathode having a surface zone of a first conductivity type in a semiconductor region at least partially surrounding the surface zone, and also relates to a semiconductor cathode produced thereby and to an electron tube produced incorporating it.
More particularly, the invention relates to such a method of providing electrical contact which results in contacts able to withstand subsequent thermal processing of an electron tube incorporating the contacted semiconductor cathode.
The method according to the invention is particularly but not exclusively suitable for semiconductor cathodes of the type commonly referred to as reverse biased junction cathodes as described, inter alia, in Netherlands Patent Application No. 7905470 in the name of the Applicant.
As described in the Patent Application, the emitting surface of the cathode is coated with a layer of a material which decreases the electron work function, preferably a mono-atomic layer of pure caesium, in order to obtain a satisfactory efficiency.
To this end the emitting surface must be cleaned in advance. This cleaning operation, which is also desirable when the layer of material decreasing the work function is not provided, is carried out after it has been mounted in the electron tube and after evacuation of the electron tube by heating the semiconductor cathode to a temperature which is sufficiently high (approximately 850.degree. C.) to remove all unwanted elements from the emitting surface.
This heating temperature is generally so high that contacts conventionally used in semiconductor technology such as, for example, aluminium, gold and silver contacts, provided by means of soldering, ultrasonic bonding or thermocompression bonding, are not resistant thereto, inter alia, because eutectic alloys or (in silicon cathodes) silicides are produced or material is attacked by melting or evaporation, leading for example, to short circuits between this zone and the surrounding semiconductor region.
Such problems notably occur if the depth of the surface zone is approximately 5 .mu.m or less.
When using contacts of materials melting at higher temperatures such as, for example, tantalum, provided by means of laser welding, such problems do not occur but the weld may become unreliable due to crack formation.